To truly take advantage of those differences in Armor Def, it takes Edef% from a Skill... A Druid's only real choice for that is a Defiance Merc.... So Since it's not, concentrate on things other than def. Def is only worth the effort if you go extreme to see actual differences.

CBF is the only thing lacking for the Merc.... but if he encounters that, you can always give him support so he's not completely crushed because he's slightly slower.